Most of the energy used in the world today is derived from the combustion of carbon and hydrogen containing fuels such as coal, oil and natural gas, as well as other organic fuels. Such combustion generates flue gases containing high levels of carbon dioxide. Due to the concerns about global warming, there is an increasing demand for the reduction of emissions of carbon dioxide to the atmosphere, why methods have been developed to remove the carbon dioxide from flue gases before the gas is released to the atmosphere.
Prior to carbon dioxide separation, the flue gas may be cleaned from other constituents and air pollutants, such as particles, SOX, NOX, mercury and water.
After purification and separation of carbon dioxide, a carbon dioxide rich stream is obtained and need to be handled, such as by storing and transportation in tanks (stationary or on a truck or ship), transporting via pipelines and/or pumping into the ground for prolonged (definitive) storage and mineralisation.
In order to reduce the amount of flue gas, and thus the dimensions of a power plant and its gas cleaning arrangements, as well as to facilitate the purification and removal of carbon dioxide, oxygen may be used instead of air in a combustion furnace, generating a flue gas with a high carbon dioxide concentration and a low nitrogen concentration. The oxygen may be obtained from separating air into an oxygen stream and a nitrogen stream by means of an air separation unit (ASU).
U.S. Pat. No. 5,931,021 discloses the liquefaction of gases with compression to supercritical pressure and the cooling by an external refrigerant to a predetermined final temperature that is lower than the saturation temperature of the liquefied gas, after which the cooled supercritical pressure gas is throttled to a prescribed subcritical pressure of the liquid. As a result, the gas is said to be liquefied without generating any flash gas.
GB 2416389 discloses the optimum values for transport and storage of liquid CO2 to be a temperature below ambient and a pressure above ambient but still sub-critical. The document focuses on how to best obtain liquid CO2 at the optimal temperature and pressure, and concludes that production of liquid carbon dioxide in bulk and at temperatures and pressures desirable for bulk transport may be effected in an environmentally friendly and efficient manner by producing liquid or dense fluid carbon dioxide at temperatures and pressures above the desired values and then expanding it to generate liquid carbon dioxide at the desired values.
EP 2 092 973 discloses subjecting CO2 from a power plant to multi-stage compression to 80-90 bar with intermittent cooling to give a fluid of 120-140° C., and then cooling to 10-30° C., after which the fluid may be pumped to 400 bar.